1. Field of the Invention
This invention relates to the Universal Service Bus (USB) communications standard that is well-known and well-regarded in the prior art and, more particularly, to the use of spread spectrum techniques to improve the electromagnetic noise characteristics of USB2.0 systems, which is not known in the prior art.
2. Description of Related Art
The USB communications standard has been created to serve as a computer data link that may be applied between computers and peripheral devices from widely differing manufacturers and sources. To serve this purpose, it has been circumscribed by highly defined standards, and any computer or peripheral device claiming to be USB-compatible must pass strict testing procedures to assure compliance with these standards. It is important to understand in detail the two specification-related measurements and validation (pass/fail) procedures. Regarding USB 2.0 standards, the following restrictions pertain:
1) The  is 480 MHz±240 KHz, that may be expressed as 480 MHz±0.05%, or ±500 ppm, and it represents an average bit speed. The signaling rate is an average. It is measured according to the USB2.0 specification by sending a fixed bit-length packet (fixed-length specific test pattern of 0's and 1's) lasting a specific length of time. The average speed is a computation result arrived at by dividing the total length of time that it takes to transmit the fixed length test packet to the number of bits in the packet. Due to physical imperfections of the transmitter, and the receiver, and of the USB2.0 cable itself, the individual test-packet bits may exceed the limits of the signaling rate. However the test-packet average must always be the within a limit of +/−500 ppm. In other words, while individual bits may violate the signaling rate limits at an individual bit-length level, the overall average frequency of the test packet must be within the +/−500 ppm specified limits. In this averaging context, a bit that has an odd duration will be averaged out, and the whole test data packet will pass the signaling limit specification of +/−500 ppm.
2) Every USB 2.0 device must pass the eye diagram test. The  is a mathematical transformation applied to each individual bit received in the test packet (each bit been measured from zero crossing to the next zero crossing). This mathematical transformation is done using MatLab™. This Eye Diagram specification of the USB2.0 bus states that no bit rising edges or bit falling edges (bit transitions) should occur inside certain “quiet” time interval also known as the .The Eye Diagram, Eye Opening interval is being described in terms of “unit interval” (UI), meaning one-bit duration. The USB2.0 specification has ample descriptions of these definitions and how to measure them. It is important to remember that, for the Eye Diagram, each individual bit transition counts and is being plotted and measured. There is no average function being performed on any of the bits.
The Eye Diagram analysis, and the tight USB2.0 Specification for the signaling rate, described above, make it obvious for any skilled professional well versed in the art that deliberately varying the 480 MHz center or average data signaling frequency will result in USB2.0 equipment failing the specification tests and such equipment being denied certification.
Thus spread spectrum techniques, which reduce electromagnetic noise spectral levels in the signal, have not been applied to the USB system. Rather, other measures have been adopted to limit or attenuate the inevitable electromagnetic noise within the system that is the inevitable and inescapable result of multiple Fourier harmonics generated by the square wave transitions of the 480 MHz signal. For example, as shown in FIG. 1, a typical USB cable 11 often includes at least one cylindrical housing 12 received about the cable adjacent to one end thereof. The housing contains a ferrite sleeve through which the cable conductor extends. The ferrite material serves as an inductive choke to attenuate some of the harmonics and limit their propagation along the cable. As a result, the USB signal is “cleaner” than it would be otherwise. However, the signal harmonics, in addition to other factors, limit the useful length of USB cables to a few meters. Beyond that limit, the attenuated signal cannot be distinguished from the EM noise with sufficient accuracy to serve as a reliable data path.